Fixing the multipart / chunking mode

This small commit fixes several issues with chunking and multipart mode.

Multipart mode failed to send file-specific POST data, while chunking did not work properly at all (after receiving one HTTP 200 OK the file was assumed to be done uploading). Neither was the response data from the server ever passed through to the to the Javascript upload succeed handler.

There may be other unresolved problems but I found it to be working in my application.

* Fix send file-specific POST data with multipart.
* Fix UploadSucceed server response data pass-through (only pass through the final response) with multipart.
* Fix file progress indication with chunking.